Battery capacity is extremely critical in many industrial, commercial and medical situations but specifically in electric motorized propulsion applications. Since charging these larger capacity batteries can take considerable time, manufacturers strive to maximize the vehicle's range. Maximizing battery capacity is one of the goals of the battery optimization system.
A battery is a grouping of at least two series connected electrical storage cells. When storage cells are connected in parallel, they are still referred to as a single cell. Identical cells are only similar in their capacity, falling within their manufacturing tolerances. It is the correction of this inequality between the capacity in substantially similar cells in a battery that is the focus of this invention.
The capacity of a battery is that amount of energy measured in Ampere-Hours (AH) between the battery's fully charged state (that point where further charge cannot be put into the battery for fear of damage occurring to any cell), and the point where the battery voltage has dropped too low (below manufacturer's specified safe levels below which the cells may be permanently damaged.)
Since the electric potential or voltage (V) of a battery is the summation of the individual cells' voltages, when any cell's voltage drops, the voltage of the battery also drops by the same amount.
When charging the battery in the conventional manner with all cells connected in series, the same amount of current I flows into each cell. Assuming all cells have the same state of charge (SOC) before charging begins, once the smallest capacity cell reaches it's maximum storage capacity, or known as a 100% SOC, the charging operation must be stopped for fear of damaging that cell. The remainder of the cells never reach their 100% SOC. When operating the battery under a load in the conventional manner, the same amount of I is drawn from each of the cells until the weakest cell reaches the point where it's voltage drops or the cell has reached it's 0% SOC. When this happens the battery voltage drops to the point of inoperability and the remainder of the cells never reach their 0% SOC. Simply stated, the battery is limited in it's ability to be charged as well as it's ability to be discharged.
In theory, if all cells could simultaneously be at the same percentage of their individual capacity at all times (equal SOC's), all cells would reach fully charged at the same time and when under load all cells would reach fully discharged at the same time. In this manner, the fullest capacity of the battery could be realized. The battery would then truly be acting as a synergistic summation of the individual cells available energy.
Battery management systems having charging systems that address charging individual cells of the battery are known in the art. These monitor parameters of the individual cells during the charging mode, and when a cell reaches full (or in general any pre-determined) storage capacity, a resistive shunt is enabled, allowing the charging current to bypass that cell while the other cells remain in the charging circuit. The drawback of this system is that all of the charging current being sent to the bypassed cell is lost to resistive heat. Further, these battery management systems do not deal with equalization of the cells in the discharge mode.
This invention attempts to keep all cells of the battery at the same state of charge (SOC) at all times, whether charging or discharging under load. It performs an initial battery and cell profiling that determines the charge capacity of each cell. From that time on, the charge capacity of each cell is closely monitored and corrected as determined necessary by the main microprocessor. Unlike conventional systems, it does not attempt to do this by lowering the SOC of the highest capacity cells by shunting excess power through resistive heat losses. Rather, it uses DC/DC converters that are enabled in conjunction with a microprocessor that monitors various cell, battery and charger parameters, such that the summation of the individual cell's energies is distributed evenly between the cells and available for utilization by the battery under load. In his manner, all cells reach a state of discharge or a state of full charge at approximately the same time and the maximum capacity for a battery to do work can be realized despite the limitations of the weakest cells.
Unlike prior art systems, the balancing of energy in independent cells is not done by comparing only voltages between individual cells, but rather by equalizing the state of charge. A cell voltage in general is poor indicator of it's state of charge, although many battery management systems use it because of it's simplicity in measurement. Having different voltages within the string of cells with known different capacities may be a desirable and normal condition depending upon the cells, and equalizing the cell voltages alone may in fact render the cells out of balance of SOC. No cells are identical and if they exhibit capacity differences within their manufacturing tolerances, having different voltages after removing equal amounts of energy is natural, normal and desired, since their remaining capacities are also different. Again, equalizing voltages in such cases will unproductively disturb this balance.
Such design innovations as the present invention provides, overcome the pitfalls of the prior art and is a cost effective, simple solution that avoids the aforementioned pitfalls of the prior art.